Micro-electromechanical system (MEMS) technology has lately been developed to form mechanical sensors of pressure, acceleration, micro switches, transducers and other micro mechanical parts and mechanical systems, by using fine processing technology of semiconductor integrated circuits.
The MEMS technology is roughly divided into bulk MEMS that is fabricated process the Si substrate itself and surface MEMS that is fabricated by deposition thin films on the surface of the Si substrate and repeating patterning processes.
In the application of the MEMS technology to sensors, mechanical transformation of a mechanism due to outside force is converted into electric signals as changes in piezoresistance or capacitance and to be outputted. And normally, the above-mentioned output is processed as signals by a semiconductor integrated circuit (LSI).
In the transducer application of the MEMS technology, the inputs and outputs signals of these transducers are connected with high frequency circuits. Thus, the MEMS structures are often used in combination with a LSI. When a MEMS is used in combination with a LSI for signal processing, each one constitutes a separate chip that making it difficult to fabricate the whole system compact. As both MEMS and LSI are normally fabricated on the Si substrate, it is natural to think of integrating them monolithically on a same substrate. This is already applied to some products, for example patent document 1, Non-patent Documents 1 and 2. These explain the method of fabricating a pressure sensor with MEMS structure in the middle of the process of manufacturing semiconductor device, to fabricate a LSI and a MEMS on a same chip.
On the other hand, the MEMS structure is not a continuous film structure from the Si substrate, unlike normal semiconductor device. Due to the presence of parts suspended in space of a single material film or laminated composite film constituting the MEMS, it is important to control the stress in the film of the MEMS structure part. It is known that, for example, if a tensile pressure is applied on a movable part independent from the periphery of which an end is fixed like a cantilever beam, the other end that is not fixed warps upward.
The Non-patent document 3 describes that, in a case of capacitance-type diaphragm, the area of a cavity part sandwiched between the upper and lower electrodes exceeds 1,500 μm2, (1) a very strong tensile stress exists on a diaphragm, the diaphragm itself is destroyed and the structure cannot be fabricated, (2) the compression stress is strong on a diaphragm, the diaphragm becomes uneven, (3) the tensile stress is within an appropriate range of 0 to 500 Mpa, it is possible to maintain the form of the MEMS diaphragm and to move it with a good linearity and sensitivity.
Generally, it is said that an effective means to maintain a form is to bring residual stress closer to zero. It is possible to control to some extent stress depending on the condition of fabricating the thin film that will be the materials for the structure and the heat treatment process subsequent thereto.
As a material for thin film MEMS structures, for example, polysilicon is widely used. Polysilicon fabricated usually at approximately 600° C. is faced with a strong residual stress. Therefore, the residual stress is released by a high temperature heat treatment of approximately 1,000° C.
In Patent Document 1, Non-Patent Documents 1 and 2, MEMS structures are with on a chip having a semiconductor circuit, poly-Si is used for the MEMS structures. MEMS structure parts are fabricated in the manufacturing process of the LSI devices, and film stress of poly-Si is released by high temperature heat treatment required for the manufacturing process of LSI.
However, CMOS LSI is fabricated only low temperature processes, about ˜450° C., to keep its high-performance. The MEMS structure is fabricated by devising a way of avoiding any damage on the LSI process, so that the MEMS structure and CMOS LSI are made on separate chips or pasted together to obtain a hybrid product.
SiGe is proposed as a low stress film to be applied for the MEMS. SiGe can be fabricated at a low temperature.
Metal alloy and silicide films, such as Cu, W, WSi and like, are also considered to be applied for MEMS structures. These material films can also be fabricated at a low temperature by sputtering. And it is possible to control film stress by adjusting the deposition conditions. FIG. 11 shows materials that can be used for fabricating MEMS in back-end of line process conveniently, after making the LSI circuit. W, Ta, Mo and other high-melting point metals are difficult to form thick films. Al is easy to corrode. Cu has many problems in the LSI process, because it is easily diffused and polluted in LSI structure and so that diffusion barrier of Cu is necessary.
Non-patent document 3 describes that the tungsten silicide (WSi) film used in the MEMS movable part. WSi is deposition at exceeding its crystallization temperature by CVD method. Temperature dependence of stress observed in the WSi film from the room-temperature to 900° C., as temperature rises. However, when the temperature returns to the room-temperature, the residual stress will be almost the same as before the measurement.
On the other hand, Non-patent document 4 describes that, WSi films is fabricated at a low temperature (150° C.), below the crystallization temperature by sputtering. It is measured that change in the film stress during heat cycle. The measured temperature is raised to 300° C., a hysteresis occurs, the film stress, after the heat cycle, shifts to the tension side as compared with before the rise (or fall) of temperature.
The MEMS structure is often located at a closer to the surface of the chip and is sometimes exposed to the outside depending on its use. Therefore various methods of protecting the MEMS structures from the outer environment have been developed. Normally, MEMS structures are sealed up. There is some cases that electromagnetic protection or shield is needed. The Patent Document 2 describes a capacitance-type pressure MEMS sensor. In this MEMS sensor, there is an electrostatic shield film on the top of the poly-Si MEMS diaphragm, and its fixed conductive film on the GND electric ground. In this case, poly-Si film stress is controlled residual by a high-temperature heat treatment. The electromagnetic shield is adhering to the MEMS or other structures, and it maybe necessary to control the residual stress of shield film.
[Patent document 1] U.S. Pat. No. 6,472,243 Specification
[Patent document 2] International Patent Laid Open WO01/014842 Brochure
[Patent document 3] JP-A No. 321612/1996
[Non-patent document 1] Klaus Kasten et al. “CMOS-compatible capacitive high temperature pressure sensors”, Sensors and Actuators 85, 2000, pp. 147-152
[Non-patent document 2] Klaus Kasten et al. “High temperature pressure sensor with monolithically integrated CMOS readout circuit based on SIMOX technology”, The 11th International Conference on Solid-State Sensors and Actuators (Munich, Germany Jun. 10-14, 2001) Collection of preliminary papers, pp. 510-513
[Non-patent document 3] T. Fujimori et al. “Fully CMOS Compatible ON-LSI Capacitive Pressure Sensor Fabricated Using Standard Back-End-of-Line Processes”, The 13th International Conference on Solid-State Sensors, Actuators and Microsystems (Seoul, Korea, Jun. 5-9, 2005) Collection of preliminary papers, pp. 37-40
[Non-patent document 4] Muh-Ling Ger, et al. “Sputtered Wsix for micro mechanical structures,” J. Mater, Res., vol. 10, No. 7, July 1995